Fluorescence bronchoscopy was carried out in 5 patients with lung cancer, using a mercury arc light source-bronchoscope system (Chest 76:27-32, 1979), 72 hours after intravenous injection of hematoporphyrin derivatve (2 mg/kg). Fluorescence was note in 3 of 5. Of the two showing no fluorescence, one was true negative and one was false negative. 15 additional patients had fluorescence bronchoscopy using a laser light source-bronchoscope system (Profio, E.A. et al, Medical Physics, November-December 1979). Distinct fluorescence was noted in 11 patients with visible endobronchial cancers. Of the 4 with no fluorescence, one was true negative, one false negative; the status of the remaining two is yet to be determined by follow-up and further studies. Our immediate aims is to demonstrate the diagnostic accuracy of fluorescence bronchoscopy in occult lung cancer (chest X-ray negative, sputum positive for malignant cells), with bronchial tumors too small to be identified reliably or even visualized by conventional white light bronchoscopy. Work in progress is aimed at developing equipment and methods to enhance detecting contrast between fluorescence of even small cancerous bronchial plaques from that of background, and with high accuracy. Pathological specimens resected will be studied to determine extent (area, thickness) and penetration of small bronchial cancers, to understand the stages of Occult Cancer, and to form a basis for therapy.